I. Field of the Invention
The present invention relates to an abrasive and, more particularly, to an abrasive suitable for removing flashes from synthetic resin molded products.
II. Description of the Prior Art
Molded products obtained through a plastic encapsulation step of semiconductor devices such as ICs or LSIs often have flashes. For example, a semiconductor molded product 11 as shown in FIG. 3 is obtained in the following manner. A lead frame 9 on which a semiconductor element (not shown) is mounted is placed in a mold and an epoxy resin is injected into the mold. Thus, a resin layer 10 encapsulates the semiconductor element on the lead frame 9. When such a molded product is obtained, resin flashes 12 tend to be formed on that portion of the lead frame 9 which is sealed by the resin layer 10 and between the leads of the lead frame 9. Thus, a step for removing such flashes is required as a post-molding step.
In order to remove the flashes 12 from the lead frame 9, a hard abrasive such as alumina, silicon carbide, or glass bead, or a soft abrasive such as a walnut shell flour is blasted onto the flashes at a high speed.
However, a problem is encountered when a hard abrasive is used. That is, such a hard abrasive has a hardness H.sub.R C of 70 or more while the cured epoxy resin which is usually employed for a semiconductor package has a hardness H.sub.R M of about 100. Thus, the abrasive is far harder than the epoxy resin and has a specific gravity which is four times that of the epoxy resin. For this reason, when the flashes 12 are removed, the surface of the molded product 11 is damaged to have a poor outer appearance. In addition to this, moisture may be introduced through the damaged portions of the molded product 11, adversely affecting reliability of the semiconductor element.
On the other hand, a problem is also encountered when a soft abrasive such as a walnut shell flour or an apricot shell flour is used. Such a soft abrasive has a large elasticity and viscosity in a dry state. Accordingly, the edges of the abrasive particles become rounded within a short period of time, and the abrasion efficiency is lowered accordingly. Furthermore, since the abrasive force of a soft abrasive is weaker than that of a hard abrasive, it will take a longer processing period of time. To shorten the processing period of time, a soft abrasive must be blasted at a higher pressure than a hard abrasive. This leads to deformation of the lead frame 9 and a higher running cost of the manufacturing equipment of the molded products. Since a soft abrasive generally is of dielectric material, when a soft abrasive is used, static electricity is generated between the abrasive and the molded product 11 upon contact therebetween. The static electricity firmly attaches the fine powder of the abrasive to the surface of the molded product 11. As a result, an outer appearance of the product 11 is degraded, leaving incomplete soldered or plated portions in a subsequent soldering or plating step. This leads to a problem of corrosion of the lead frame 9.